It is possible for a light beam to make multiple dispersive passes with respect to a dispersive medium, but for simplicity, a double-pass case illustrates the relevant principles.
FIG. 1 illustrates schematically a current optical spectrum analyzer (OSA) instrument (see Agilent Technologies data sheet 8614xB Optical Spectrum Analyzer Family Technical Specifications), incorporating a double-pass monochromator 10 having diffraction grating 17 as a dispersive element. An input beam 12a typically entering through input fiber 11, which can serve as an input aperture, is directed by first mirror 13a and second mirror 13b through collimating element 16 in two passes 12a and 12c onto diffraction grating 17. Between the two passes, the beam is directed through an intermediate resolution defining aperture that is normally incorporated into slit wheel 14 to provide a range of aperture sizes. Collimating element 16, for example a lens, refocuses diffracted light from the surface of grating 17 in each pass 12b and 12d back into the optical plane of slit wheel 14. Output beam 12e is deflected by output mirror 13c into output fiber 18, which can function as an output aperture of monochromator 10, such that it reduces stray light in the system and, due to diffraction and coupling effects, improves resolution bandwidth compared to a single-pass monochromator.
Referring to the coordinate axes in FIG. 1, the grating dispersion direction is parallel to the y-axis, which is perpendicular to the plane of the figure, whereas the non-dispersion direction is parallel to the x-axis, pointing upward parallel to the plane of the figure. Both x and y axes are mutually perpendicular to the z-axis, which is essentially parallel to the effective propagation direction of light beam passes 12a, 12b, 12c, and 12d. There are two varieties of double-pass monochromator, the additive or double-dispersive monochromator, and the subtractive or re-condensing monochromator. Each of these architectures provides specific advantages depending on the application. In an additive monochromator, the orientation of the wavefront of light impinging on dispersing component 17 is the same, for example plus y, in both first pass and second pass. Because the light beam is focused between two passes 12a and 12c through intermediate slit or aperture system 14 (where the wavefront both in and perpendicular to the plane of dispersion is naturally inverted to −y, −x), the additive architecture requires introduction of an additional top-to-bottom inversion (−y to plus y) of the wavefront in the plane of dispersion between the first contact and the second contact with dispersing element 17. In current instruments, component 15 typically represents a half-wave plate that rotates the polarization of the light beam by 90 degrees between first pass 12b and second pass 12c. Alternatively (or additionally), component 15 illustratively represents a top-to-bottom inverter (−y to plus y) of the wavefront, which, when inserted in second pass 12c immediately after reflection from second mirror 13b, converts monochromator 10 from subtractive to additive architecture. This results in the dispersion in second pass 12c being additive to the dispersion in first pass 12a (both plus y or both −y), producing a distinct narrowing in system resolution of a double-pass monochromator compared with that of a single-pass monochromator.
In the prior art, depicted by subtractive monochromator 10, simply by focusing the light through the intermediate slit in slit wheel 14, the wavefront is inverted top to bottom (−y) relative to the orientation of first pass wavefront orientation (plus y) at dispersing element 17. In general, it is not necessary to further invert the wavefront, as the required wavefront inversion is already accomplished, such that a separate wavefront-inverting component illustratively represented by component 15 is not needed. The spectrally dispersed and filtered (by intermediate slit at slit wheel 14) light from first pass 12b is spectrally recombined by dispersing element 17 in second pass 12c. The result is a spectrally uniform and highly compact focal spot at the monochromator exit aperture or slit 18. Since the light is already spectrally dispersed and prefiltered at intermediate slit 14 in the first pass, system resolution for a two-pass subtractive monochromator is substantially the same as system resolution of a corresponding single pass monochromator.
Additionally, all path lengths of the light transmitted through a subtractive monochromator are made equal by having inverted the wavefront top-to-bottom. For example, in a reflective diffraction grating implementation, diffraction grating 17 is tilted to align the desired wavelength to intermediate slit 14, thereby introducing a wavelength-dependent path length. For example, the beam of light impinging on the grating surface encounters path lengths varying monotonically with position along the length of the grating in the plane of dispersion, resulting in a range of path lengths corresponding to the range of wavelengths transmitted through output slit 18. Inverting the wavefront between passes cancels this path length difference and hence the temporal dispersion that results from path length difference. The additive monochromator architecture does not cancel this path length variation, but rather effectively doubles it in the second pass.
In an optical spectrum analyzer (OSA) employing a monochromator as its optical engine, a conventional operational procedure is to sweep a reflective diffraction grating (or other dispersive component) in angle in order to obtain the spectra of various optical wavelengths. The spectrally dispersed light is propagated onto a variable width output slit and is sensed in an analog fashion by a single finite area photodiode or similar detector. The speed at which an OSA can determine spectral information derived from a photodetector signal is contingent on many factors, including the physical size (active area) of the photodiode. Small area photodiodes are generally faster and have a direct impact on the speed of an OSA. The subtractive monochromator engine will provide an output spot of filtered and re-combined spectra that can be focused within a small detector active area. An additive monochromator produces a highly spread or dispersed spot on the output slit. This requires a large area detector to collect the exiting light. In addition to being slow, the large area of the detector is inclined to collect unwanted or “stray” light. This necessitates some sort of noise suppression, for example a chopping system, to minimize optical noise. Such accessories add both expense and complexity. On the other hand, an additive monochromator has a significantly better (narrower wavelength spread) resolution, which is desirable in certain applications.
A narrower resolution in a multiple-pass monochromator is generally useful only when the slits (both internal and exit) are positioned near their narrow settings. In such circumstances, even an additive monochromator would function well with a small area photodiode. It is when the slits are enlarged that the small area detector will not capture the entire incident cone of output light from an additive monochromator. Given these considerations, if a component could be added to a monochromator architecture that would allow the architecture to switch reversibly between additive and subtractive, then the functionality could be additive and achieve the narrowest resolution bandwidth on the smallest slit, and could revert to a subtractive configuration for all wider slits in order to increase speed. This maximizes OSA measurement speeds at all slit widths, while providing the opportunity for the narrowest resolution bandwidth on the smallest slit, which is the only filter setting where an improvement in resolution bandwidth offers substantial value.